Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, Indiana 46202, USA.
Nature. 2011 Jul 3;475(7355):240-3. doi: 10.1038/nature10162.
Mediator is a key regulator of eukaryotic transcription, connecting activators and repressors bound to regulatory DNA elements with RNA polymerase II (Pol II). In the yeast Saccharomyces cerevisiae, Mediator comprises 25 subunits with a total mass of more than one megadalton (refs 5, 6) and is organized into three modules, called head, middle/arm and tail. Our understanding of Mediator assembly and its role in regulating transcription has been impeded so far by limited structural information. Here we report the crystal structure of the essential Mediator head module (seven subunits, with a mass of 223 kilodaltons) at a resolution of 4.3 ångströms. Our structure reveals three distinct domains, with the integrity of the complex centred on a bundle of ten helices from five different head subunits. An intricate pattern of interactions within this helical bundle ensures the stable assembly of the head subunits and provides the binding sites for general transcription factors and Pol II. Our structural and functional data suggest that the head module juxtaposes transcription factor IIH and the carboxy-terminal domain of the largest subunit of Pol II, thereby facilitating phosphorylation of the carboxy-terminal domain of Pol II. Our results reveal architectural principles underlying the role of Mediator in the regulation of gene expression.
中介体是真核转录的关键调节因子，将结合在调控 DNA 元件上的激活子和抑制子与 RNA 聚合酶 II（Pol II）连接起来。在酵母酿酒酵母中，中介体由 25 个亚基组成，总质量超过一兆道尔顿（参考文献 5、6），并组织成三个模块，称为头部、中间/臂和尾部。到目前为止，我们对中介体组装及其在转录调控中的作用的理解一直受到有限的结构信息的阻碍。在这里，我们报告了必需的中介体头部模块（七个亚基，质量为 223 千道尔顿）的晶体结构，分辨率为 4.3 ångströms。我们的结构揭示了三个不同的结构域，复合物的完整性以来自五个不同头部亚基的十个螺旋的束为中心。螺旋束内复杂的相互作用模式确保了头部亚基的稳定组装，并为一般转录因子和 Pol II 提供了结合位点。我们的结构和功能数据表明，头部模块使转录因子 IIH 和 Pol II 最大亚基的羧基末端结构域并列，从而促进 Pol II 羧基末端结构域的磷酸化。我们的结果揭示了中介体在基因表达调控中的作用的结构原则。
